1. Field of the Invention
The present invention relates to a process for continuous preparation of a reduced-color isocyanurate-functional polyisocyanate prepared from 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI).
2. Description of the Background
For high-quality one- and two-component polyurethane coating materials of high light stability and weather resistance, the isocyanate component employed comprises, in particular, polyisocyanate mixtures comprising isocyanurate groups and uretdione groups. These products are preferably prepared by catalytic oligomerization of (cyclo)aliphatic diisocyanates, examples of which are 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI) and 1,6-diisocyanatohexane (HDI).
Catalysts which can be employed to facilitate the oligomerization reaction include tertiary amines, phosphines, alkali metal phenolates, aminosilanes, quaternary ammonium hydroxides or quaternary ammonium carbonates. Other highly suitable oligomerization catalysts include hydroxides, halides and carboxylates of hydroxyalkylammonium ions; alkali metal salts; and the tin, zinc and lead salts of alkylcarboxylic acids. Depending on the catalyst the use of various cocatalysts is also possible such as OH-functionalized compounds or Mannich bases formed from secondary amines and aldehydes and/or ketones, for example.
The (cyclo)aliphatic diisocyanates are oligomerized by allowing them to react in the presence of the catalyst, with or without the use of solvents and/or auxiliaries, until the desired conversion has been reached. The reaction is then terminated by deactivating the catalyst and the excess monomeric diisocyanate is removed by distillation. Deactivation takes place by means of heat or by adding a catalyst inhibitor to the reaction. Depending on the type of catalyst used and on the reaction temperature, the resulting polyisocyanates have varying proportions of isocyanurate and/or uretdione groups.
The majority of the products prepared in this manner are clear, but depending on the type of catalyst, quality of diisocyanate, temperature of reaction and mode of reaction they may show a more or less pronounced yellow coloration. For the preparation of high-quality polyurethane coating materials, however, it is important that the products have an extremely low color number.
Organic polyisocyanates such as aromatic, cycloaliphatic and aliphatic polyisocyanates with a functionality of two or more, can be prepared by various methods (Annalen der Chemie 562 (1949), pages 75ff). One method which is particularly established in industry is the preparation of organic polyisocyanates by phosgenating organic polyamides to give the corresponding polycarbamic chlorides and thermally cleaving these chlorides into organic polyisocyanates and hydrogen chloride. This method of preparation has been utilized exclusively in industry for a long time.
Problems associated with this procedure are the high conversion of chlorine to hydrogen chloride by way of phosgene and carbamic chloride, the toxicity of the phosgene and the associated cost-intensive safety measures, the corrosiveness of the reaction mixture, the lability of the solvents commonly employed and the formation of chlorine-containing and chlorine-free byproducts, which are codeterminants of the physical properties of the product such as the color, viscosity and vapor pressure, and of the chemical properties, such as reactivity and storage life of the polyisocyanates. The known phosgenation products of aniline-formaldehyde condensates (crude polyisocyanate mixtures of the diphenylmethane series), for example, include a large number of impurities. According to Chem. Soc. Rev. 3 (1974) page 209 ff., these impurities principally comprise chlorine-containing contaminants, which always cause fluctuations in activity when the chlorine involved is "highly mobile", so-called hydrolyzable chlorine.
With the objective of circumventing the problems associated with chlorine, numerous experiments have been conducted to prepare organic polyisocyanates without the use of phosgene, i.e. phosgene-free processes. According to EP 0 126 299 (U.S. Pat. No. 4,596,678), EP 0 126 300 (U.S. Pat. No. 4,596,679) and EP 0 355 443 (U.S. Pat. No. 5,087,739) it is possible to prepare (cyclo)aliphatic diisocyanates such as 1,6-hexamethylene diisocyanate (HDI) and/or isomeric aliphatic diisocyanates having 6 carbon atoms in the alkylene radical and 1 -isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (isophorone diisocyanate or IPDI) in circulation processes, by reacting the (cyclo)aliphatic diamines with urea and alcohols and with any N-unsubstituted carbamic esters, dialkyl carbonates and other byproducts recycled from the reaction process to give (cyclo)aliphatic biscarbamic esters and then thermally cleaving these esters into the corresponding diisocyanates and alcohols.
The diisocyanates HDI and IPDI prepared by the phospene-free process will be referred to below as HDI (urea) and IPDI (urea), respectively.
Like the products of the phosgene process, the polyisocyanates prepared by a chlorine-free technique are also subject to problems. It is reported that (cyclo)aliphatic polyiso-cyanates obtainable by the phosgene-free process, especially by thermal cleavage of (cyclo)aliphatic polycarbamic esters, are not stable on storage (EP 0 645 372). Their instability is attributed to the absence of hydrolyzable chlorine compounds and to the presence of catalytic impurities of unknown structure that promote, for example, the formation of oligomers. At low temperatures, for example at .+-.5.degree. C. and below, hexamethylene diisocyanate (HDI (urea)), for example, tends to form linear HDI oligomers having a nylon 1 structure. The resulting increase in molecular weight, which is associated with an increase in viscosity, may lead to the gelling of the polyisocyanate, e.g. HDI (urea). Products of this kind can no longer be reacted reproducibly to give polyisocyanate polyaddition products. At higher storage temperatures, for example, the reactivity of HDI prepared by phosgene-free processes, especially in the case of the trimerization reaction catalyzed with quaternary ammonium hydroxide compounds, decreases sharply. Intensely colored, isocyanurate-functional polyisocyanates are obtained which can no longer be utilized especially as a base material for coating.
Both processes for preparing organic polyisocyanates, i.e., both the phosgenation process and the phosgene-free process via polycarbamic esters, therefore, give products which are problematic when further processed into isocyanurate-functional polyisocyanate mixtures as are employed in high-quality one- and two-component polyurethane coating materials. The cause of this lies in preparation-related byproducts of, in many cases, unknown structure, or else in a preparation-related deficit of certain byproducts which influence the shelf life, reactivity and color of the composition and whose effects extend into corresponding successor products, thereby hindering reproducible and hence economic use.
Isocyanurate-functional polyisocyanate prepared from 1-isocyanato-3,5,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI) is obtained industrially in a continuous process by partial trimerization of IPDI and subsequent separation of the excess monomer by short-path evaporation (EP 0 017 998, U.S. Pat. No. 4,454,317). The trimerization takes place in the presence of quaternary ammonium carboxylates.
For the technical trimerization of IPDI the grade of IPDI employed to date has been that as obtained by phosgenation of isophoronediamine (DE 12 02 785). This standard IPDI contains 100-400 mg/kg of total chlorine, of which the hydrolyzable chlorine content is 80-200 mg/kg. The final product, which has been freed from monomer, is obtained as a solid resin. 70% strength solutions of this resin in butyl acetate are clear and have a pale yellow coloration. The color numbers of such solutions lie within a spectrum of between 70 and 150 Hazen, although similar color numbers are obtained in solvents with aromatic components. If production is continued over a relatively long period, two principal phenomena are observed: firstly, there is a continuous buildup within the production unit of unwanted deposits of undefined composition, which necessitate regular cleaning of the unit, a process which is evidently neutral neither in terms of time nor cost. Secondly, the color quality of the product is subject to gradual impairment, in other words, the color numbers show a trend toward higher values along the time axis. Such a trend, however, is disadvantageous, since only products having an extremely low color number are desired for the preparation of high-quality polyurethane coating materials and coatings. A need continues to exist for an improved method of preparing low colored isocyanurate-functional polyisocyanate.